Friable abrasive media

ABSTRACT

The present invention describes a novel composition and system for use for removal of organic and inorganic materials from a variety of substrates, including a tooth surface, without damage occurring to the substrate. The disclosed composition comprises a friable abrasive particle which when propelled by a gas stream at a surface for a time sufficient departs adequate energy to remove organic and inorganic materials from the surface without causing any significant ablation of the surface.

The present invention describes a novel composition and system for usefor removal of organic and inorganic materials from a variety ofsubstrates without damage occurring to the substrate. The disclosedcomposition comprises a friable abrasive particle which when propelledby a gas stream at a surface for a time sufficient departs adequateenergy to remove organic and inorganic materials from the surfacewithout causing any significant ablation of the surface.

One area of use for this invention is within the field of dentistry,with particular value in the special of orthodontics. For years dentistshave straightened and repositioned teeth with the use of tensioningwires affixed to teeth via attachment brackets. As is well known in theart, brackets are commonly secured directly to the tooth with adhesiveresins and cements. When the brackets are removed either intentionallysuch as at the end of the treatment, or unintentionally due to a bondfailure, the adhesive components remains both on the tooth and thebracket. The adhesive is then removed from the tooth structure bymechanical means such as scraping, ultrasonic scaling, or grinding witha bur. These methods of resin removal are not only time consuming butcan also be damaging to the tooth. As the curette or bur scrapes closeto the tooth surface, gouges and scratches are almost always left in theenamel. When gouges or scrapes occur, additional time is spent trying topolish out the scratches and healthy enamel is necessarily removed. Evenwhen the current techniques result in removal of the resin from thetooth surface without scratching the tooth, the enamel pellicle is neverreally cleaned. This failure to remove all matter from the pellicle canbe aesthetically unpleasant and limit the effectiveness of toothwhitening.

After debonding a bracket, removing resin paste from a tooth is oneproblem. The next problem is resin removal from the bracket; that is ifit is to be reused. For most metal brackets sandblasting with aninstrument as in Fernwood, U.S. Pat. No. 4,941,298 with a commonly usedalpha aluminum oxide or silicon carbide particle works well. Fornon-metallic or ceramic brackets having a specially fused silica bondingface, the hard aluminum oxide abrasive sandblasting will not workbecause it quickly removes the fused silica facing. The bracket thenbecomes useless unless a new silica facing is placed on the bracket.Sandblasting with the disclosed composition allows brackets treated witha silica facing to be cleaned for reuse and rebonded. A furtherorthodontic application is the sandblast cleaning of the adhesiveresidue remaining around the edges of brackets when they are firstbonded onto the tooth. Prior to this invention, the excess residue wouldnormally be scraped off the tooth and around the bracket with aninstrument such as a curette.

A further dental use is found for the simple and non damaging removal oftemporary resins and cements from tooth structure.

A still further use in the field of dentistry is suggested forexpediting the whitening of teeth. Teeth whitening in dentistry isgenerally performed by applying a whitening chemical for a period oftime. The invention described herein speeds teeth whitening by removingall organic and inorganic contaminants in the tooth enamel pelliclethereby aiding in the delivery of the tooth whitening agent directly tothe tooth. Various other examples for application in dentistry areenvisioned where it is desirous to remove any organic or inorganicmaterial from the tooth structure without damaging the tooth structure.

A still yet further dental use is within the area of implant dentistry.Implants are sometimes coated with a biocompatible coatings such ashydroxyapatite or alternatively a silica coating. The disclosedcomposition and system for use cleans debris from the coated implantswithout removing the coatings.

Outside the field of dentistry, the disclosed invention may be used fora variety of other applications. In the field of commercial painting,the invention discloses an improved way of paint removal withoutdamaging the underlying structure. Other applications are found in theareas of art restoration, jewelry work and fossil restoration whereorganic and inorganic contaminants are to be removed without resultantdamage to the underlying structure. Another application is found in theremoval of contaminants on electronic components such as cleaningcomputer drive disk whether optical, floppy or metallic removal ofmasking material on silicon wafers, or circuit boards.

PRIOR ART

In the field of dentistry various methods exist which attempt to solvethe problems previously discussed. For cleaning of organic and inorganicmaterials from tooth enamel sandblasting with "soft" (Dolomite, Mohhardness 4 to 6 instead of 8 or 9 for Alpha Alumina) insoluble abrasivethrough sandblasting means was disclosed by Black, U.S. Pat. No.3,972,123; thereafter, Gallant, U.S. Pat. No. 4,174,571 proposed usingsoluble abrasives, particularly sodium bicarbonate, through sandblastingmeans to clean teeth. Although the latter approach is still commerciallyutilized, the system is inherently erosive to tooth structure and caremust be used to avoid removing excessive tooth structure during cleaningprocedures. These systems are generally not used for resin adhesiveremoval but rather for stain removal and general tooth cleaning.

For removing resin adhesives during orthodontic procedures, or forinlays/onlay cement removal, devices such as those disclosed in Blackand Gallant, have been used with little or no success, rather devicescommonly referred to as ultrasonic scalers such as the Cavitron byDentsply, dental burrs or simply hand instruments such as scrapers, arethe commonly used means. The later devices do successfully remove theresinous materials, however significant care must be used to avoidsevere erosion occurring to the tooth structure. Further, resin removalby hand instruments are relatively slow and time consuming.

For applications other than dentistry, sandblasting with plastic andother pliant abrasive media, Lynn, U.S. Pat. No. 5,207,034 has beenknown to provide a means of removing organic and inorganic materialssuch as paint from structures without damaging the underlying structure.Two of the limitations of the plastic media methods known to theinventors have been the commercial price and the potential toxicity.

SPECIFICATION

Described more fully herein is a friable abrasive composition and systemfor its use for removing organic and inorganic materials from asubstrate. Commonly used sandblasting abrasive media such as alphaaluminum oxide, silicon carbide and others generally are categorized assolid, homogeneous, relatively non-friable abrasives and range inhardness from 6 to 10 on the Moh hardness scale. The Moh index is one ofthe relative indexes used to indicate relative hardness. Moh 1 ischaracterized as extremely soft whereas Moh 10 which is specified asdiamond is the hardest material known. These abrasives range in hardnessdepending upon the desired result and work performed by transferring thekinetic energy of the accelerated particles to the surface. The physicsof sandblasting with particles as just described (aluminum oxide,silicon carbide) is simply a chipping and fracturing of a surface byhigh-speed particles impinging upon it. As long as the impingingparticles are as hard or harder than the surface they're hitting,ablation will occur along with removal of any surface contaminants.There are at least five physical characteristics of an abrasive thataffect its performance as a sandblasting medium; 1) particle velocity,2) particle mass, 3) particle shape, 4) hardness, and 5)toughness alsosome times referred to as friability. Just as there are abrasioncontrolling characteristics of the propelled abrasive particles, sothere are concomitant characteristics of the abraded surface whichgreatly affect the outcome. The factors include; 1)hardness, 2)surfacetoughness and 3)surface malleability.

The physical mechanism of sandblasting with the novel particlesdescribed herein varies widely from traditional sandblasting operations.The following generally describes the unique and patentable operation ofthe disclosed composition and system for use. The disclosed compositionis comprised of large (10 to 200 micron) "snowball" like spheres. Like asnowball, the spheres are comprised of hundreds to thousands of tiny(0.01 to 5 micron) particles. The tiny particles are loosely boundtogether, allowing the spheres to crumble when they impinge upon theirtargeted surface. As a result of this impact crumbling, three mechanismsare observed. First, the spheres break apart and the tiny particlessplay over a finite area as they disjoin. Second, each tiny particlequickly loses its kinetic energy as the splaying-out process continues.Unlike traditional sandblasting, there is essentially no bounce back bythe spheres as compared to traditional abrasive media of comparable sizeand hence the bulk of their kinetic energy is transferred to thetargeted surface rather than conserving their kinetic energy andrebounding. This is the significant difference between traditionalsandblasting with hard, non-friable particles and sandblasting with thespheres described herein. Lastly, the energy transfer to the surfaceshows up as heat. Experimentally, we measured surface temperatureincreases in the range of 25 degrees Celsius. This heat transfer isextremely significant when the desired material to be removed is heatsensitive, such as may be the case with a dental resin. Experimentally,we observed that the outer surface of a dental adhesive or resin willslightly soften with this instantaneous but tiny amount of heat andpieces of it will dislodge quickly. As each successive sphere strikesduring sandblasting, the adhesive/resin surface is softened and removed.When the last of the resin/adhesive is removed from the tooth enamel, wenoted experimentally and confirmed by scanning electron microscopy thatessentially no ablation of the tooth surface occurs at pressures as highas 200 psi for periods in excess of two minutes. Two reasons are thoughtto explain this phenomena; first, the tooth enamel, Moh hardness 4.5 isharder than the spheres, Moh hardness 3.5 and secondly, the convectioncooling offsets the heat imparted to the enamel. When we used Dolomite(Moh hardness 3.5), a traditional abrasive, with similar hardness wewere able to remove the dental resin however we noticed some toothenamel erosion. The importance of having an abrasive which will notdamage the underlying structure is particularly important in the fieldof dentistry where by law only the dentist is able to perform procedureswhich involve tooth structure removal. With the disclosed composition, adentists assistant can remove dental resins without fear of toothstructure damage.

We have observed a similar phenomena when we used the disclosed abrasiveon temporary resin cement attached to tooth dentin as opposed to toothenamel. In general the tooth enamel hardness is 4.5 on the Moh hardnessscale as compared to dentin which is 2.5 on the Moh hardness scale.After the temporary resin cement is removed from the dentin, we didhowever observe slight ablation of the dentin after two minutes ofsandblasting at pressures up to 200 psi. The slight ablation is believedto have occurred because the disclosed abrasive is slightly harder thanthe tooth dentin. Based upon the observed dentin ablation, it isimportant to offer multiple hardnesses of the disclosed friable abrasiveto eliminate any potential dentinal damage. The method of varying spherehardness is hereinafter disclosed.

Sphere hardness can be modified somewhat to allow softer or hardermaterials to be efficiently cleaned. To illustrate, we performed paintremoval on two different metals, aluminum and steel. We choose anon-alloyed aluminum having a hardness of 2 to 2.9 Moh and a steel witha hardness of approximately 5 Moh. We applied the same enamel paint toboth surfaces. After the paint was dry we removed the paint with thedisclosed composition (3.5 Moh). We observed no damage to the steelstructure and only very minor ablation of the aluminum structure. Wethen did the same test with Dolomite (3.5 Moh) and observed minorablation of the steel and moderate to severe ablation of the aluminum.Thereafter we used a slightly softer, Moh 3.0 variant of the disclosedcomposition and still observed no damage to the steel and now observedeven less damage to the aluminum. Their was no observed timedifferential between the Moh 3.5 and Moh 3.0 compositions to remove thepaint. Based upon our experimentation, the disclosed system isparticularly effective with heat sensitive contaminants, such as resins,plastics, paints. All of the various applications can not possibly bediscussed herein, and those trained in the various relevant arts willobserve additional consistent uses.

Spheres of the agglomerated small particles may be chosen from the groupof aluminum trihydrates, such as gibbsite, bayerite and nordstrandite.These naturally occurring minerals when ground and sized in the range of10 to 200 micron spheres will work effectively for the novel systemdisclosed. The raw material, for example gibbsite, may be ground andsized to a specific particle micron range (0.01 to 5 micron). It is thenpressed into a cake and sintered at a temperature below the transitionpoint or below the point where water of hydration is lost. The cake isthen reground and sized into spheres ranging in size between 10 and 200microns for packaging and shipping. Variations of this basic processcould involve adding water or a binder after the first grinding. In thiscase, the sintering step might be altered or even eliminated. For dentalapplications, gibbsite works well but is not the only choice. Any smallparticle abrasive that can be processed into spheres and with a hardnessless than Moh 4.0 would produce the same effect. Choices includedicalcium phosphate, gypsum, calcium carbonate, dolomite, as well asmany other suitable similar particles. Aluminum Trihydrates made fromGibbsite are commercially available from Alcoa as plastic and paperfillers and work suitably for the disclosed system. Spheres made fromother minerals are not known to be commercially available. The novelcompositions disclosed herein are the manufacture of spheres other thanaluminum trihydrates comprising a plurality of tiny particles boundtogether to form larger sphere like particle with varying hardness. Thealuminum trihydrates range in hardness between 2.5 and 3.5 Moh. Sphereswith differing desired hardness can be constructed out of suitable 0.01to 5 micron particles generally having a hardness equal or up to 5 Mohless than the targeted structure. The constituent particles however mayvary greatly in hardness depending upon desired ablation levels to thetargeted structure.

In order to remove materials faster and where some moderate level ofablation is permissible, the disclosed composition may be mixed withtraditional abrasive media such as alpha aluminum oxide.

When the spheres disjoin upon impact with the targeted surface, the tinyconstituent particles cause a small dust cloud. The dust created ismessy and unpleasant. Consequently, it is envisioned that a particleevacuation means operate concurrently to the sandblasting to allow formore pleasant and efficient system.

Now having disclosed the composition and system for use, we claim:
 1. Amethod of removal of organic or inorganic material from tooth enamelwithout erosion of said tooth enamel, which method comprises:directing afriable abrasive composition suspended in a gas stream against saidtooth enamel for a time sufficient to remove at least a portion of saidorganic or inorganic material, said friable abrasive compositioncomprising spherical particles of aluminum trihydroxide of size 10-200microns which upon impact with said tooth enamel disintegrate intofragments of size 0.01-5 microns.
 2. The method of claim 1, wherein thealuminum trihydroxide particles have a Moh hardness of less than about4.0.
 3. The method of claim 2, wherein said particles impacts said toothenamel in a stream of air.